[Bill S (OP)]

https://www.nature.com/news/the-new-thermodynamics-how-quantum-physics-is-bending-the-rules-1.22937?WT.ec_id=NATURE

I'v not read much of the article yet; but it does seem to raise some interesting possibilities.

[evan_au]

Back when I was in high school, our science teacher said that it wasn't clear if the laws of thermodynamics applied in 2 extreme scenarios:
- At the level of the individual quantum
- At the level of the whole universe

It looks like researchers are making some progress towards quantifying thermodynamics in the quantum realm.
There has been some progress towards quantifying thermodynamics at the level of a black hole
But it looks like we are still guessing at the level of a whole universe...

[puppypower]

The second law is about entropy, with entropy being a state variable. Any given state of matter has a given amount of entropy, which is constant for that specific state. For example, liquid water at 25C and 1 atmosphere has an entropy of 188.8 joules/(mole K). This is the same no matter who measures it. This value for entropy, for this specific state of matter, is always the same, over time.

The second law states that the entropy of the universe needs to increase. All this means is change needs to occur into new states of higher entropy, for the universe as a net whole. This is due to the speed of light being the ground state of the universe. I have explained this elsewhere. Although the need to increase entropy is true for the universe as a whole, which is evolving into new states, smaller subsets of the universe, can lower entropy into previous states, as long as the sum of all states in the universe continues to go up.

Gravity is interesting, because gravity is the largest natural mechanism that lowers entropy. The pressures generated by gravity, on our atmosphere can condense vapors into liquids of lowered entropy. Or, in the case of the core, what should be gaseous iron at 5000C, becomes a solid at the gravity induced pressures of the core,

The question then becomes, with all this entropy lowering due to gravity, how does the universe stay ahead so there is no violation of the second law? The expansion of the universe is the main way. There is an action and reaction due to the entropy potential induced by gravity.

An interesting application of entropy, is connected to space-time. We know gravity causes space-time to contract. In terms of time, clocks slow as space-time contracts. In the black hole time has nearly stopped relative to our reference. Say we had two identical factories that make widgets. Each factory makes defects at the rate of 1 per hour. This new state, called defect, has a higher entropy and reflects the second law in action.

If one factory was placed in a space-time reference, similar to the surface of the earth and one factory was placed in a space-time reference similar to the core of the earth, since time moves slower in the core reference, the rate of entropy generation; generation of defects slows, relative to the surface reference and factory, since one hour take longer in the core; side by side.

As such, although gravity lowers entropy, the parallel contraction of space-time moderates this by slowing the rate of entropy lowering.  One might argue the contraction of space-time is also connected to entropy potential and the needs of the second law. While the expansion of the universe, by speeding up time, accelerates the production of countering entropy; faster rate of widget defects, so the second law continues to dominate.

[jeffreyH]

What about a scalar like temperature in a remote frame with extreme time dilation? If we were to use their clocks instead of our own to make measurements in their frame then their entopy is also 'dilated'.

[jeffreyH]

Taking this a stage further, as time dilation approaches infinity (as at the event horizon of a black hole), the change in entropy must approach zero. This means that black holes must halt the progress of entropy. It could be thought of like a reset. So the reason why entropy would be low at an event like the big bang is due to the strength of the combined forces. I can't validate this of course.

[Bill S (OP)]

A possibly relevant quote from Sean Carroll: “From Eternity to Here”:

  “Before Boltzmann, the second law was absolute – an ironclad law of nature.  But the definition of entropy in terms of atoms comes with a stark implication entropy doesn’t necessarily increase, even in a closed system; it is simply likely to increase.”

[Bill S (OP)]

So the reason why entropy would be low at an event like the big bang is due to the strength of the combined forces.

Is it necessary to insist on a low-entropy start for the Universe?

The FLRW model describes the Universe in its first instant as a single quantum, containing all the matter and energy of the Universe, and completely filling the tiny space it occupied.  When it expanded, it was not a case of the matter/energy exploding and moving outward.  Its contents remained stationary relative to space, but become further apart as the expansion took place.  In that first “quantum” a quark/gluon plasma occupied all the available space; there was no room for manoeuvre; entropy could not have started evolving until more space became available. 

Surely, that would mean that entropy must have been at its maximum; it could increase only when expansion provided the potential for increase; so we don’t have to account for a mysteriously low entropy start. 

[Zer0]

Curiosity's sparking a fire of violation it seems,  🗽

Trying to redirect the arrow of time n direction of light beams.  🔛

Beware thy actions would resurrect n summon,  🌋

Either Laplace's or Maxwell's but surely a demon.  😈

[evan_au]

Is it necessary to insist on a low-entropy start for the Universe?

When I was at school, a cyclic universe was considered a real possibility. If everything in the universe were crammed back into the "cosmic egg" for another cycle, it's not clear that entropy would have increased, over the cycle.

However, with more recent discoveries of the accelerating expansion of the universe, the possibility of a cyclic universe now seems very remote.

[jeffreyH]

I tend to agree with Evan on the cyclic universe idea. I am at the moment just letting the Heisenberg picture of quantum mechanics sink in. I wish I had looked into this years ago. It's like the light bulb just went on. Much better than wave mechanics.
I am so glad I studied linear algebra.

[Bill S (OP)]

I tend to agree with Evan on the cyclic universe idea.

Over the course of a few years I have passed through phases of shifting opinion about cyclic universe and multiverse ideas; including reaching a point at which I reasoned that if there were other universes, and if time were "created" with each, there would be no time outside each, and every, universe; therefore, there would be no difference between multiple co-existing universes and cyclic universes. 

However, the thought that there can never have been nothing does tend towards acknowledging that time must have preceded the "creation" of the Universe, so there would be a difference.

As I see it (at the moment) the multiverse could exist without violating the second law, but I doubt same could be true of the cyclic universe.